1. Technical Field
The present invention relates to a sheet metal component and a method for manufacturing the same, and more particularly to, a sheet metal component which permits easy and reliable inspection of its weld quality and a manufacturing method thereof.
2. Related Art
Vehicular components such as a frame and a cross member affecting the safety of a vehicle are in some cases are constructed of sheet metal components. Some of these sheet metal components are obtained by welding two metal sheets at their confronting ends and by press-forming a resultant one-piece metal sheet into a desired shape. For example, a side frame of a vehicle is constituted by a front member comprised of a first metal sheet and a rear member comprised of a second metal sheet whose thickness and mechanical strength are larger than those of the first metal sheet, so that it may be deformed to absorb the impact upon head-on collision. The first and second metal sheets are subject to butt welding or lap welding, to be united with each other at their confronting ends, and the resultant one-piece metal sheet is subject to press forming to be formed into a side frame of a desired shape to which important security parts and/or specially guaranteed parts such as suspension unit, engine, transmission unit of a vehicle are attached. Thus, a weld failure caused in the side frame can result in a problem of safety of the vehicle.
In order to determine the weld quality in respect of important sheet metal components, such as side frame, greatly affecting the safety, it is advisable to carry out the total inspection of these components at the stage just after the components are fabricated or after entire cars each incorporating therein the sheet metal component are obtained. Thus, methods for carrying out such total inspection have been investigated.
The weld quality of a sheet metal component can be inspected accurately, if the component can be cut at its weld portion so that the cut portion may be subject to visual inspection for weld quality determination in term of, e.g., a magnifying glass. Although this method is applicable to a sampling inspection, it is of course impossible to apply such method to a total inspection. On the other hand, a weld quality determination of a sheet metal component such as a side frame cannot be made in term of a visual inspection at the stage of obtaining an entire car whose side frame is already painted.
An object of the present invention is to provide a metal sheet component having a weld portion whose weld quality can be determined easily and accurately in term of appearance inspection and a method for manufacturing such a metal sheet component.
According to one aspect of the present invention, there is provided a sheet metal component comprising a main body which is comprised of one-piece metal sheet obtained by welding two metal sheets at their confronting ends, and an expanding portion which projects from a surface of the main body at a welded portion of the main body and which has an external appearance varying in dependence on a weld state of the welded portion.
Preferably, the expanding portion projects from the surface of the main body by a predetermined amount which is set so as not to produce an unusual appearance, indicative of weld failure, in the expanding portion if the two metal sheets are welded properly at the welded portion and so as to produce the unusual appearance if they are improperly welded.
According to the sheet metal component of this invention, the expanding portion has its external appearance varying in dependence on the weld state of the welded portion of the one-piece metal sheet from which the sheet metal component is fabricated. That is, if the weld state (weld quality) is proper, no unusual appearance such as crack is caused in the expanding portion. On the other hand, if improper welding is made, the expanding portion has such an unusual appearance. Based on the appearance of the expanding portion, therefore, the weld state of the sheet metal component can be determined properly. In this manner, the sheet metal component of this invention permits an easy and accurate determination of the weld quality of the welded portion based only on the appearance inspection. For this reason, the present invention is especially useful to provide a sheet metal component such as an automotive frame to which an important security part or a specially guaranteed part is attached and the weld quality of which affects the safety. Since sheet metal components whose weld quality can be determined positively by means of appearance inspection before and after they are assembled into final products, without the need of cutting the sheet metal components at their welded portions, they can be subject to a 100% inspection or total inspection. For instance, in the case of automotive side frames according to this invention, their weld qualities can be easily and accurately determined in total inspection at one or both of the stages before and after they are respectively assembled into vehicle bodies, whereby the quality assurance and the safety of vehicles can be improved.
In the present invention, the two metal sheets may have different shapes, e.g., different thicknesses. The two metal sheets may be butt-welded or lap-welded at their confronting ends. With these preferred sheet metal components, the two metal sheets are welded into the one-piece metal sheet having a required mechanical property and a desired shape which are suited to mechanical and appearance requirements of the sheet metal component. For instance, it is possible to provide an automotive side frame in which a front member, one of the two metal sheets, is greatly deformed when an impact force is applied thereto, thereby absorbing the impact force.
In the present invention, preferably, the expanding portion is formed into a semi-spherical shape, and has its top formed in alignment with part of the welded portion. With this preferred arrangement, the expanding portion has its external appearance more accurately represents the weld state of the weld portion, and therefore, a more accurate determination of the weld state in term of appearance inspection can be carried out.
According to another aspect of the present invention, there is provided a method for manufacturing a sheet metal component in which two metal sheets are welded at their confronting ends to obtain a one-piece metal sheet which is press-formed into the sheet metal component with use of a first pressing machine.
The manufacturing method comprises the steps of (a) transferring the one-piece metal sheet, which is already press-formed or is not press-formed as yet, between first and second press die halves of a second pressing machine and positioning a metal sheet section of the one-piece metal sheet between a punch formed in the first press-die half and a recess formed in the second press-die half so as to face the punch and permit the punch to intrude thereinto, the metal sheet section including part of a welded portion of the one-piece metal sheet, and (b) causing the punch to press the metal sheet section of the one-piece metal sheet so as to subject the metal sheet section to plastic deformation, to thereby form an expanding portion in the one-piece metal sheet, the expanding portion having its external appearance varying in dependence on a weld state of the welded portion.
Preferably, the step (b) includes causing a predetermined amount of plastic deformation in the metal sheet section, to thereby form the expanding portion. The predetermined amount of plastic deformation is set so as not to produce an unusual appearance, indicative of weld failure, in the expanding portion if the two metal sheets are properly welded and to produce the unusual appearance if they are improperly welded.
With the manufacturing method of this invention, a sheet metal component whose weld state is determinable by appearance inspection can be manufactured properly. Since the sheet metal component has the expanding portion which has an external appearance thereof varying in dependence on the weld state, the weld state can be properly determined based on the external appearance of the expanding portion.
Incidentally, the present invention was created by taking note of phenomenon which takes place in a test piece during Erichsen test in which a spherical punch is gradually pressed against a plate material (test piece) and the depth of pressing the punch observed just when a crack appears in the plate material is determined as an Erichsen value for the evaluation of formability of the plate material.
As distinct from the Erichsen test in which the test piece is subject to plastic deformation until a crack actually appears in the test piece, the present invention forms the expanding portion by subjecting the metal sheet section, including part of the welded portion, of the one-piece metal sheet to plastic deformation until a predetermined amount of deformation is caused in the metal sheet section. The predetermined amount of plastic deformation is restricted to the extent that no crack appears in the expanding portion if the weld state of the welded portion of the one-piece metal sheet is proper as usual.
In the present invention, preferably, the steps (a) and (b) are carried out, with use of the second pressing machine which also serves as the first pressing machine, at the same time when the one-piece metal sheet is press-formed into the sheet metal component of a desired shape, whereby the expanding portion is formed simultaneously when the one-piece metal sheet is press-formed. With this preferred method, the efficiency of manufacturing the sheet metal component having the expanding portion can be improved, and in particular, sheet metal components subjected to total inspection in respect of weld quality can be manufactured efficiently.
Alternatively, the expanding portion is formed in the one-piece metal sheet with use of the second pressing machine, exclusively used for formation of the expanding portion, other than the first pressing machine. The formation of the expanding portion is made before or after the one-piece metal sheet is press-formed. According to this modified method, a sheet metal component with or without the expanding portion can be manufactured where required. Thus, a sampling inspection in respect of the weld quality of a large number of sheet metal components can be made by providing some of them with expanding portions and subjecting the same to inspection based on external appearances of the expanding portions.
Preferably, the two metal sheets are butt-welded or lap-welded at the welded portion.
Preferably, the steps (a) and (b) are carried out by using the punch formed into a semi-spherical shape, and the expanding portion is formed into a semi-spherical shape.
Preferably, the step (a) includes positioning the one-piece metal sheet so that the part of the welded portion is in alignment with a distal end of the punch, and the step (b) is carried out in this positioning state, thereby forming the expanding portion whose top is formed in alignment with the part of the welded portion.
Preferably, the second press-die half does not apply a force onto the metal sheet section as the metal sheet section is subject to the plastic deformation within the recess of the second press-die half by being pressed by the punch of the first press-die half in the step (b). With this preferred method, the expanding portion having its external appearance representing the weld state of the welded portion more accurately can be formed.